In recent years, a sealed-type rechargeable battery (which may be hereinafter simply referred to as “rechargeable battery”) represented by a lithium ion rechargeable battery is used as a power source not only for a mobile apparatus such as a portable phone or a notebook personal computer but also for an electrically driven vehicle such as an electric automobile or a hybrid car. A cell constituting the rechargeable battery has a structure in which an electrode group is accommodated in an inside of a sealed outer casing, and the electrode group is constructed by winding or stacking a positive electrode and a negative electrode with a separator interposed therebetween. For example, a laminate film such as an aluminum laminate foil is used as the outer casing.
For use that requires a high voltage such as in a power source for electrically driven vehicles, the rechargeable battery is used in a mode of a battery module including a plurality of cells. In the battery module, the plurality of cells constitute an assembled battery in which the cells are connected, for example, two in parallel by two in series, and are accommodated in a case. Generally, the battery module mounted in vehicles is used in a mode of a battery pack. In the battery pack, a plurality of battery modules is connected in series and is accommodated in a case together with various apparatuses such as a controller. The case of the battery pack is formed to have a shape suitable for mounting in a vehicle, for example, a shape that conforms to the underfloor shape of the vehicle.
the rechargeable battery has a problem that, when the electrolytic solution is decomposed due to overcharging or the like, the cell swells to be deformed in accordance with rise in the internal pressure caused by the decomposition gas and, unless the charging current or the discharging current is stopped, this results in rupture of the rechargeable battery. Therefore, in order to prevent troubles, it is important to detect deformation of the rechargeable battery with a high degree of sensitivity so that the charging current or the discharging current can be stopped at appropriate times. Furthermore, in the rechargeable battery during the charging or discharging, the electrode group expands or contracts by change in volume of the active substance, so that a technique capable of detection at a high precision by distinguishing between the swelling accompanying such charging or discharging and the swelling accompanying the gas generation is desired.
Patent Document 1 discloses a method in which, by using a strain gauge provided on a wall surface of a battery case, the swelling of the battery case is detected. However, the sensitivity region of such a strain gauge is comparatively narrow. Also, to the swelling accompanying the gas generation (which may hereafter be referred to as “gas swelling”) that increases with time, the swelling accompanying the charging or discharging (which may hereafter be referred to as “electrode swelling”), which is a smaller deformation than the gas swelling, is added. Therefore, it seems impossible to detect with a high precision by distinguishing between these swellings with the strain gauge.
Patent Document 2 discloses an apparatus in which, with respect to two or more thin-type batteries included in an assembled battery, a magnetic force generated by an electric current that flows through a bus bar in which the respective terminals thereof having the same polarity are connected is detected, and an abnormality is detected based on the detected magnetic force. However, in this method, the swelling of the thin-type batteries is not directly detected, thereby raising a fear that the sensitivity may be insufficient in detecting deformation of the rechargeable battery caused by swelling of the cells.
Patent Document 3 discloses a system in which, with respect to a side surface of a cell case that faces in a stacking direction of a stacked body of electrodes and a side surface of the cell case that faces in a direction perpendicular to the stacking direction, inter-axial distances between these two side surfaces and a wall surface of an accommodating case or the like are respectively detected, and an internal-pressure abnormality of the cells is detected based on the both detection results. However, in order to detect these inter-axial distances, a plurality of piezoelectric elements must be provided for one cell case, thereby unavoidably making the structure complex.